A color pipeline maps color input values to halftone data for printing. The color input values corresponds to an image or color. The color input values may include a colorimetric value in a color space, for example defined in CIE XYZ or CIE Lab, or a device dependent model such as RGB or CMYK. The color pipeline maps such color input values to halftone data, using color management, color separation, linearization and halftoning. The color pipeline includes a color separation table that specifies the ink vector data corresponding to the color input values. This ink vector data results in a halftone pattern as a result of the predetermined halftoning algorithm.
For setting up such pipeline, a number of halftone patterns is assigned to a number of colors that a certain print system can print, via said color separation, linearization and halftoning. This number of halftone patterns comprises a relatively small set of halftone patterns that is optimal for the specific print system. The assignment can be made on the basis of printed and measured halftone patterns. From the small number of halftone patterns a mapping for the full color gamut is generated.
While this mapping sometimes works well, in most cases non-optimal halftone patterns are assigned to the colors in the gamut. Moreover, for a specific pipeline, a specific mapping method is usually applied. It is however known that in many cases one process may be better suitable for one color range, while another process may be better suitable for another color range. Optimally, different halftoning algorithms are applied.
Sometimes better halftone patterns are known for a specific print system, while the installed system pipeline may not have them incorporated. Such better halftone patterns may be obtained systematically or even manually by trial and error. For example, such halftone patterns have shown to result in optimal ink usage for specific colors, or give better results for other print attributes such as smooth transitions between colors, low cost per copy, high color constancy, and low grain. The better result that is achieved by these optimized halftone patterns is not achieved by the print system having the non-adapted pipeline installed. The same accounts for available optimized ink combinations that achieve good results. These ink combinations may be expressed as halftone pattern. At present, these are also difficult to incorporate in the pipeline.
One way to try to integrate halftone patterns with a print system pipeline is to post-process the halftone patterns by interpolation. A halftone pattern assigned to a chosen color is introduced in the color separation table by replacing the corresponding ink vector and then propagating the corresponding changes to nearby ink vectors in the color separation table. Another known way consists of performing multiple color separations, using one or more color separations for the newly introduced halftone data, and other color separations for other halftone data, and subsequently merging the performed color separations.
However, in both of these methods the color relationship between an ink vector and a corresponding color is non-linear. Also, the relationship between an ink vector and a color depends on each different ink, how these inks overprint and the substrate.
One of the objects of the invention is to better facilitate incorporation of known pairs of halftone patterns and color values in a print system pipeline.